Various methods may be employed to reduce the intake noise of an internal combustion engine. One method is to use a Helmholtz resonator on an intake air pipe configured to communicate intake air to the internal combustion engine. The intake air pipe is typically disposed upstream from an intake manifold and is configured to communicate intake air to the intake manifold of the internal combustion engine. A Helmholtz resonator includes a resonance volume or chamber having a small opening, typically referred to as a neck. The neck is operable to enable communication between the resonance chamber and the intake air pipe. Sound waves generated by components within the internal combustion engine travel along the intake air pipe where their acoustic pressure impinges on the neck. This acoustic pressure excites a mass of air within the neck. The acoustic pressure within the resonance chamber reacts against the air mass within the neck and produces an out-of-phase acoustic pressure at the intake air pipe to cause cancellation of intake noise at the resonant frequency. In this way, some of the engine noise is eliminated as the out-of-phase acoustic pressures in the intake air pipe cancel each other.
The frequency at which the attenuating acoustic pressures reach their maximum amplitude is known as the resonant frequency. A number of parameters determine the resonant frequency and bandwidth of a Helmholtz resonator, including the volume of the resonance chamber and the length and cross sectional area of the neck. Minor changes to the length and cross sectional area of the neck may alter the resonance frequency and thereby reduce the effectiveness of the Helmholtz resonator.